Well I'm not a dev, but I can tell you, because it's the same reason that they're difficult to build in real life - you have no yaw stability. The vertical stabiliser on a plane normally stops the plane from yawing uncontrollably, and a flying wing doesn't have one. On planes like the B-2, this is solved by using decelerons, which are like two ailerons which open on the top and bottom of the same wing to provide yaw authority. Another way to describe a deceleron is that it's an airbrake off to one side of the plane. Now in a modern flying wing, the computer system controls the decelerons to maintain yaw stability. In SimplePlanes, we don't have decelerons or fly-by-wire, so we have to work around it. The two easiest options are hiding vertical stabilisers (see my Taranis) or using a large dihedral on the wing (my Martlet). Sorry for rambling on so much, I hope this helps.